How to make a bind to Ctrl Key for toggling snapping on/off
Unfortunately, you can't use solely a modifier for a shortcut, plus there already is a shortcut to toggle this: Shift + Tab.



If you still want to, you can change this in the User Preferences.

If you open the user preferences with Ctrl + Alt + U, and switch to the Input tab, you can search by shortcut (Click the dropdown next to the search box and select Key-Binding) to find entries containing "shift tab":



Now, find the entry under 3D view:



And press where it says Shift Tab and press the keys you want to bind.